The present invention relates to a spread spectrum (SS) communication system, and more particularly, to a spread spectrum communication system wherein pseudo noise codes to be used in despreading received signals at a receiving (demodulating) side, are generated using a circuit of relatively simple arrangement, thus dispensing with a spread code generator at the receiving side.
Communication techniques in Japan have recently undergone remarkable development with a variety of system techniques. Multiplex operation, typically in optical communication techniques is one such development. A main trend in radios communication techniques is multi channel access (MCA) for effective utilization of analog modulated radios. Although the MCA system is advantageous in that a specific channel is not occupied long time by one user, the number of channels is limited and hence the number of communications which can be performed at the same time does not exceed it. Therefore, a user in large cities where channels are busy, is often urged to wait for his or her turn. The MCA system uses a limited frequency domain by dividing only its bandwidth. It is considered that such a system nowadays stands to loose the requirements of current information fidelity and instantaneousness.
More efficient communication systems have been studied and developed by many researchers. One of them is a spread spectrum communication system developed about fifty years ago. The fundamental concept thereof was established in the nineteen fifties. In the nineteen sixties, a pseudo noise code generator made of as many transistors as one hundred had been used. However, the active elements thereof and the manufacturing methods at that time were not still mature, and it has long been desired to improve them. With recent developments for high density IC techniques and low-cost ICs, more compact and inexpensive devices have become available so that the spread spectrum communication system has been receiving attention once again.
According to the spread spectrum communication system, a carrier is primary modulated by an information signal, and the primary modulated carrier is secondary modulated by wide band noise-like spread codes to spread it over a very wide bandwidth. The spread spectrum communication system generally includes a direct spread (DS) system, frequency hopping (FH) system, hybrid system and the like, depending upon the secondary modulation scheme. The present invention concerns the DS system for this. The spread spectrum communication system has a number of advantages as in the following:
1. High security and privacy;
2. Strong resistance against external interference and noise, and against intentional disturbance;
3. Compatibility with conventional systems;
4. No control station and control channel as in an MCA station;
5. Capability of supervising by using address codes;
6. Seemingly no radio because of low power density of the DS system (Capability of transmitting radio with thinly spread power);
7. Capability of increasing the number of stations in trade for a slight degradation of speech quality; and
8. Capability of multiplex operation at a same frequency band by changing pseudo noise codes.
Taking the above advantages into consideration, the spread spectrum communication system is now widely applied to not only communication fields but also to various other fields such as consumer apparatus.
With the above-described technical background kept in mind, the fundamental principle of spread spectrum communication and conventional spread spectrum communication will be described next.
Referring to FIG. 1 showing the basic structure of a conventional spread spectrum communication system, a station A generally indicated by numeral 1 at the transmission side has a primary modulator 2 for receiving a carrier and an information signal and performing primary modulation, a spread code generator 3 for generating spread codes to be used for spread modulation of the primary modulated signal F1, a spread modulator 4 for receiving the primary modulated signal F1 and a spread code signal Fn and performing secondary modulation, and an antenna A1 of the A station 1. In FIG. 1, the signal Fn represents a radio wave transmitted from an ordinary station which becomes an interference station to the transmission side A station 1 and a receiving side B station 5.
The receiving side B station 5 has a spread code generator 6 for generating spread codes to be used for despreading a received signal, a despreading circuit 7 for despreading the received signal from an antenna A2 in accordance with the spread codes, a filter 8 for passing the frequency components of preferably a specific narrow band of an output from the despreading circuit 7, and a demodulator 9 for demodulating an output from the filter 8.
Next, the function of a conventional spread spectrum communication system will be described with reference to the frequency spectrums shown in FIG. 2 at each circuit portion of FIG. 1.
The primary modulation signal F1, as shown in FIG. 2(a), which was primarily modulated at the primary modulator 2 at the transmission side A station 1, is secondarily modulated at the spread modulator 4 by the spread code signal Fss (refer to FIG. 2(b)) from the spread code generator 3 and amplified to be output from the antenna A1 as the transmitted signal F1s. The type of primary modulations is not particularly limited, but frequency modulation (FM), phase shift keying (PSK) and the like may be adopted (in this specification, the description is assumed to use PSK modulation). For the secondary modulation (spread modulation), PSK modulation is generally performed using pseudo noises (hereinafter called PN code). It is required for the PN code to be like a random noise as much as possible, and to have a predetermined period for picking up the code at the receiving side.
Next, the function of the receiving side will be described. At the receiving side B station 5, the transmitted signal F1s obtained from the antenna A2 via a filter and high frequency amplifier (not shown) is despread at the despread circuit 7 using a despread code from the despread code generator 6. The despread code generator 6 is in synchronization with the spread code generator 3 of the A station, and the same PN code (Fss) is used. Radio waves incoming to the antenna A2 include not only an F1s radio wave but also F2s, F3s, . . . , radio waves from other SS stations, and radio waves Fn from general stations other than SS stations, as shown in FIG. 2(c). The spectrum shown in FIG. 2(a) is restored from the radio wave F1s being dealt with, and shown in FIG. 2(d), at the filter components other than F1s (refer to FIG. 2(e)). Then, original information is demodulated at the demodulator 9. As seen from FIG. 2(e), the output signal from the filter 8 includes not only F1s but also interference radio waves Fn and a fraction of SS radio waves from other SS stations. The ratio of the object signal power to the residual power is called the DN ratio (a ratio of desired signal power to interference power). In order to obtain a larger DN ratio, the spread bandwidth should advantageously be as wide as possible. The spread bandwidth is generally set at 100 to 1000 times as wide as the frequency bandwidth of an information signal or the primarily modulated signal.
The basic principle of the spread spectrum communication has been described before. Next, the particular operation will be theoretically described for primary and secondary modulation and demodulation during the spread spectrum communication. A spread spectrum signal S(t) (F1s shown in FIG. 1) for the spread spectrum communication is given by the following equation, taking the information data as d(t) [+1 or -1], the spread code Fss as P(t) [+1 or -1], and the carrier as cos.omega..sub.c t: EQU S(t)=d(t)P(t) cos.omega..sub.c t (1)
(where .omega..sub.c =2 .pi.fc)
At the receiving side, the incoming spread spectrum signal S(t) is transformed into a two-phase PSK signal d(t)cos.omega..sub.c t by multiplying (or correlating, despreading) it by the spread code P(t). The spread code P(t) (actually P(t) with slight delay is obtained, in synchronization with the spread code associated with the spread spectrum signal at the time of transmission, by using a spread code clock derived from the incoming spread spectrum signal. The obtained two-phase PSK signal is multiplied by the reproduced cos.omega..sub.c t (actually cos.omega..sub.c t with slight delay) for synchronous detection to obtain: EQU d(t)(cos.omega..sub.c t).sup.2 =d(t)(1+cos2.omega..sub.c t)/2
After removing the double carrier component 2.omega..sub.c t by the filter, the information data d(t) are demodulated.
The process gain Gp of the spread spectrum communication is given by: EQU Gp=Bp/B.sub.D ( 2)
where B.sub.D is the bandwidth (main lobe of the spectrums) of the two-phase PSK signal d(t)cos.omega..sub.c t, and Bp is the band width (main lobe of the spectrums) of the spread spectrum signal spread by the spread code P(t). The process gain Gp is several hundreds to several thousands for the case of an ordinary design. Suppression of interference signals, noises and the like depends upon the process gain. Thus, the wider the frequency bandwidth of the spread spectrum signal is set with respect to the information data d(t), the interference signals, noises and the like can be suppressed to the larger extent. Such improvement therefore depends substantially and exclusively upon the process gain Gp.
In the spread spectrum communication system, "despreading" at the receiving side is most important. However, it is difficult to generate a reliable spread code required in performing such despreading. Despreading methods presently adopted in general include a despreading method using an AFC loop, delay locked loop and multiplier, and a despreading method using a synchronous loop of matched filter and multiplier.
However, the above-described conventional despreading methods for a spread spectrum communication system pose the problem of having a complicated circuit arrangement, problems associated with circuit adjustment, and cost effectiveness. These problems must be solved for allowing various applications to consumer apparatus.
Furthermore, a conventional communication system requires a very wide frequency band so that it poses a problem associated with the effective utilization of the frequency band (radio waves). The allowable frequency band is limited in practical use so that it is difficult to design the communication system as desired.